1. Technical Field
The present disclosure relates to apparatus and methods for imaging using a scanning electron beam. More particularly, the present disclosure relates to dark field imaging using a scanning electron beam.
2. Description of the Background Art
The two most common types of electron microscopes available commercially are the scanning electron microscope (SEM) and the transmission electron microscope (TEM). In an SEM, the specimen is scanned with a focused beam of electrons which produce secondary and/or backscattered electrons as the beam hits the specimen. These are detected and typically converted into an image of the surface of the specimen. Specimens in a TEM are examined by passing the electron beam through them, revealing more information of the internal structure of specimens.
Bright field imaging and dark field imaging are often used in the context of TEMs. A bright field image may be formed in a TEM by selecting electrons from a central diffraction spot to form the image. A dark field image may be formed in a TEM by selecting some or all of the (non-central) diffracted electrons to form the image. The selection of electrons may be implemented using an aperture into the back focal plane of the objective lens, thus blocking out most of the diffraction pattern except that which is visible through the aperture. Dark field imaging is typically less commonly used in SEMs than in TEMs.
It is desirable to improve electron beam instrumentation for bright field and dark field electron imaging.